The relationship between the development of delayed hypersensitivity and acquired antituberculous or anti-typhoid immunity will continue to be investigated using the mouse infection model. Comparison of the extracellular protein antigens released by logarithmically growing cultures of H37Rv and H37Ra will be continued using the superinfection test system. Protein antigens isolated from S. typhi, S. enterititidis and S. paratyphi cultures will be tested for DTH responses in actively infected animals. The effect of T-cell depletion on the development of these DTH responses, as well as on the ability of the host to resist a Salmonella challenge will be determined. Attempts to vaccinate mice with the purified Salmonella antigen by introduction into a BCG stimulated lymph node will be compared with the level of DTH and CMI induced by 2 doses of the antigen suspended in Freund's complete adjuvant. Finally, more extensive studies will be made of the effect of "thymosin" treatment of T-cell depleted mice so far as the ability of the animal to mount an effective immune response to a Mycobacterial challenge. The effect of thymosin dosage and the treatment regimen on the level of antituberculous resistance will be quantitated in terms of the behaviour of the BCG populations in the lungs and spleens of normal and T-cell depleted mice. Bibliographic references: Morrison, N.E. and Collins, F.M. 1975. Immunogenicity of an aerogenic BCG vaccine in T-cell depleted and normal mice. Infec. Immun. 11, 1110-1121; Collins, F.M., Montalbine, V. and Morrison, N.E. 1975. Growth and immunogenicity of photochromogenic strains of Mycobacteria in the footpads of normal mice. Infec. Immun. 11, 1074-1087.